1. Field of the Invention
This invention pertains to the general field of interferometry and apparatus for testing surfaces. In particular, it provides a novel disk-mount device for successively and automatically testing computer-drive disk blanks for quality control purposes in an assembly line.
2. Description of the Prior Art
As computer equipment becomes smaller and subject to wear and tear associated with portability, the precise flatness of drive disks becomes more and more critical to high-speed performance and reliability. Moreover, any scratch or other imperfection on the surface of a disk is cause for discarding it during manufacture. Thus, conventional aluminum disks are increasingly being replaced by glass disks, which are lighter and less subject to warpage and, therefore, preferable for most applications. The flatness of both kinds of disks is typically tested for quality control by interferometric measurements that identify damaged or substandard areas on either of the two parallel surfaces of a disk. For complete quality control, the flatness of one surface of the disk is first measured on an instrument such as a Fizeau interferometer and the disk is then flipped over to test the opposite surface. During high-volume manufacture, though, disk blanks are processed at a rate of many thousands per hour; therefore, tests are often performed on a limited number of samples from each batch and only on one side of the disk. For example, current disk production needs require the ability to test about 3,600 blanks per hour.
As well known in the art, the surface geometry of an aluminum or glass computer-drive disk can be measured with a Fizeau-type interferometer with reference to a flat reference surface (known in the art as a transmission flat). As illustrated in schematic representation in FIG. 1, the interferometric device 30 comprises a light source 10 (normally a laser operating in the single mode) producing a beam of light 12 that is passed through a microscope objective 14 and a spatial filter 16, such as a pinhole. The light 12 is then collimated by a very-well corrected collimating objective 18 and directed through a transmission flat 20 (comprising the reference surface 21) toward the test surface 23 (consisting of one side of a computer-drive disk 22) positioned collinearly (with respect to the light beam) and substantially in parallel to the reference surface at some distance within the coherence length of the light source 10. The light reflected by the test surface 23 interferes with the light reflected at the reference surface 21 and, according to the principle of superposition, bright interference fringes are produced corresponding to all points on the reference surface where the optical path difference (OPD) of the light is equal to a multiple of its wavelength. A beam splitter 24 is placed between the spatial filter 16 and the collimating objective 18 in order to reflect the fringes to the side, so that they may be observed on a screen or directed to a camera 26 through appropriate lenses 28 for display, and/or to other instrumentation for recording and data processing. The interference fringes so produced are used to provide a measure of the flatness of the tested disk surface.
Prior-art equipment for testing disk blanks utilizes mounting devices that permit the placement of the disk 22 on a sample stage in a desired predetermined spatial relation with respect to the transmission flat 20. It is essential that the disk surface mounted on the stage be positioned as desired and then held still for interferometric measurements. It is also essential that each successive disk placed on the mount device be fixed in the same exact position. Appropriate mechanisms for adjusting the tip and tilt of the disk surface are also provided, as well known in the art, operating on the back surface of the disk, rather than directly on the test surface.
Pressure applied by the mounting mechanism may cause disk deformation, which in turn results in misalignment with respect to the reference surface. In addition, vibrations in the disk mount can cause measurement errors. Accordingly, these devices are not very stable while performing repeated measurements. U.S. Pat. No. 5,689,337, herein incorporated by reference, discloses a self-aligning disk mount consisting of a hub capable of retaining a disk in precise alignment with the transmission flat by hanging its center hole on two support posts and resting the disk on three pressure tips on the hub. A system of push-pull adjustment screws is provided to set the proper tip/tilt of the disk. The plane established by the three pressure tips provides a fixed, aligned plane against which the test surface of successive disks is directly positioned in alignment with the reference surface. By utilizing gravity to position the disk against the three pressure tips in substantially vertical disposition, deformation of the disk is minimized.
All prior-art testing equipment is designed for manual feed of disk blanks and is not suitable for high-volume on-line testing. Even devices that ensure the alignment of successive disks placed in test position, such as the apparatus described in the '337 patent, require that each disk be handled in and out of the retaining structure that holds the disk in place during the test procedure. This requirement slows the operation down and renders current equipment unsuitable for automatic, high-speed testing. Therefore, there is still a need for a more versatile solution.